Seismic data acquisition and processing techniques are used to generate a profile (image) of a geophysical structure (subsurface) of the strata underlying the land surface or seafloor. Among other things, seismic data acquisition involves the generation of elastic waves, the collection of reflected/refracted versions of those elastic waves and processing to generate the image. This image does not necessarily provide an accurate location for oil and gas reservoirs, but it may suggest, to those trained in the field, the presence or absence of oil and/or gas reservoirs. Thus, providing an improved image of the subsurface in a shorter period of time is an ongoing process in the field of seismic surveying.
The generation of seismic images from seismic surveys is a time consuming and expensive undertaking, i.e., the longer it takes to collect and process the seismic data, the greater the cost of the seismic survey. In another aspect, longer seismic survey times may have a greater environmental impact, which is a consideration when requesting permits to perform a seismic survey. Further longer seismic survey times may lead to greater hazardous exposure to individuals operating the seismic survey equipment. Accordingly, one of the ways to reduce the cost of generating seismic images, the environmental impact and the hazardous exposure is to decrease the time required to collect seismic data. Among other things, this can be accomplished by firing multiple seismic sources simultaneously or in a time overlapping manner to reduce the time needed to collect seismic data. However when firing multiple sources simultaneously or in a time overlapping manner, it then becomes necessary to reassign the received elastic wave energy to its originating source by separating the sources during processing of the data.
In order to provide some context for the subsequent embodiments for separating sources, consider a data acquisition process and system as will now be described with respect to FIG. 1. FIG. 1 shows a system 100 for the land-based acquisition of seismic data. The system 100 includes a plurality of receivers 102 positioned over an area 104 of a subsurface to be explored and in contact with the surface 106 of the ground. A number of sources 108 are also placed on the surface 106 in an area 110, in a vicinity of the area 104 of the receivers 102. A recording device 112 is connected to the plurality of receivers 102 and placed, for example, in a station/truck 114. Each source 108 can be composed of a variable number of vibrators, typically between one and five, and can include a local controller 116. A central controller 118 can be provided to coordinate the shooting times of the sources 108. A GPS system 120 can be used to time-correlate the sources 108 and the receivers 102. With this configuration, sources 108 are controlled to generate seismic waves, and the plurality of receivers 102 records waves reflected by the oil and/or gas reservoirs and other structures in the subsurface.
Considering conventional approaches to reduce the time required to generate seismic images using systems like that illustrated in FIG. 1, and for at least the reasons described above, groups of seismic sources 108 are often fired in overlapping (in time) patterns creating blended signals for recording by the receivers 102. Subsequent processing of the recorded seismic data separates the sources such that the sources appear to have been fired sequentially, however in order to perform this separation processing the overlapped firings may require the use of sources with different frequency characteristics or predefined spatial and/or temporal separation between the source firings. Additionally, factors associated with the local sample area can impact the required source firing separation parameters described above and the source separation strategies, sensitive to signal distortion, can fail to achieve the desired level of separation.
Accordingly, it would be desirable to provide systems and methods that avoid the afore-described problems of separating overlapped source firings without requiring complex firing schedules and layouts and which provide a shorter period of time for conducting a seismic survey.